Wound line returned sound (6)
I believe the answer is:
lesion
'wound' is the definition.
(I've seen this before)
'line returned sound' is the wordplay.
'line' becomes 'l'.
'returned' says the letters should be written in reverse.
'sound' becomes 'noise' (noising is a kind of sounding).
'noise' written backwards gives 'esion'.
'l'+'esion'='LESION'
(Other definitions for lesion that I've seen before include "Damaged area of tissue" , "Harmful change in bodily tissue as from injury" , "Kind of injury in insole" , "Damage to tissue from injury or disease" , "Abrasion" .)